Industrially practical processes of catalytic hydrogenolytic dealkylation of aromatic hydrocarbons typically include processes in which a reaction is conducted at a high temperature between 600.degree. C. and 650.degree. C. in the presence of a chromia-alumina catalyst as disclosed, e.g., in U.S. Pat. No. 2,951,886. These processes primarily aim at demethylation of toluene, uses of which, except for gasoline and a solvent, are restricted, to produce benzene useful as a raw material of nylon synthesis.
While no noteworthy improvement has ever been added to these processes, Neftekhimiya, Vol. 15, No. 1, p. 95 (1975) reports that demethylation of toluene for production of benzene can be carried out at a temperature lower than the conventionally employed range, i.e., between 400.degree. C. and 500.degree. C., by using a catalyst comprising a novel metal of the Group VIII, e.g., ruthenium, rhodium, palladium, osmium, iridium, and platinum, supported on an alumina carrier.
However, since the novel metal-on-carrier catalysts need further improvements in activity, durability, reaction selectivity, and the like, many proposals have been made, for example, a combined use of two or more novel metals and addition of other transition metal components.
For example, JP-A-58-210851 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a catalyst having improved low-temperature activity, which is obtained by impregnating rhodium into an alumina carrier followed by calcination in an oxygen atmosphere, and JP-A-58-210851 proposes a catalyst having improved low-temperature activity and prolonged duration, which is obtained by supporting iridium and an alkaline earth metal on an alumina carrier.
These improved catalysts are still unsatisfactory in terms of catalytic activity, duration, and reaction selectivity when applied to heavy aromatic hydrocarbons containing 8 to 10 carbon atoms.
On the other hand, known crystalline methallo-silicate catalysts for reactions of aromatic hydrocarbons include those for disproportionation (JP-A-52-65229), those for isomerization (JP-B-53-41658, the term "JP-B" as used herein means an "examined published Japanese patent application"), and those for alkylation (JP-B-56-44050), and some of them have been put to practical use. A crystalline metallo-silicate catalyst predominantly catalyzing dealkylation of aromatic hydrocarbons is unknown.
The above-mentioned processes conventionally adopted on an industrial scale are associated with serious problems such that (1) the reaction temperature is as high as 600.degree. to 650.degree. C., (2) even starting with any alkyl-substituted aromatic hydrocarbon other than toluene which has been used as a main starting material of dealkylation, e.g., ethylbenzene, dimethylbenzene (xylene), propylbenzene, and methylethylbenzene, the reaction results in production of benzene in a major proportion, with surprising low selectivity to toluene or xylene which is a useful chemical raw material, and (3) reduction in catalytic activity is more noticeable as the starting aromatic hydrocarbon becomes heavier.
While the reaction temperature can be dropped to 500.degree. C. or less by using the above-described novel metal-on-alumina catalyst, the literature affords no working example of starting with heavy aromatic hydrocarbons having 8 to 10 carbon atoms. Moreover, the problems relating to low reaction selectivity and high rate of activity reduction have not yet been settled.